1. Field of the Invention
The present invention relates to a contactless optical writing apparatus for recording information on a rewritable thermal recording medium, the apparatus enabling recording and erasure of information in a contactless manner without direct contact with a heating device such as a thermal head.
2. Description of the Related Art
There is a thermal recording system in which a diazo compound-based heat-sensitive material is utilized. There are reversible thermal recording paper and the like that enable repeating of color development and color disappearance at a specific temperature. In the thermal recording paper, color development and color disappearance take place by heating by means of a heating device such as a thermal head. As a recording system for such thermal recording paper, there is a system in which a recording head such a thermal head is brought into direct contact with the thermal recording paper. In this system, the recording head is brought into direct contact with the thermal recording paper, and hence the following problems are brought about. For example, wear and stain of the recording head are easily caused. Further, the printing surface of the thermal recording paper is rubbed and stained. The service life of the recording head is shortened due to a short circuit caused by an accretion or excessive power supply or the like.
On the other hand, as a technique of information recording using thermal recording paper, there are techniques disclosed in, for example, Japanese Patent No. 3266922 and Japanese Patent No. 2561098. Japanese Patent No. 3266922 relates to a method of developing and disappearing a color in a contactless manner by using a reversible heat-sensitive material, and discloses an information recording medium in which an infrared absorbing layer that absorbs infrared rays to generate heat and a thermal recording layer are stacked in sequence on a substrate. Of these layers, the thermal recording layer is constituted of a heat-sensitive color development layer or a metallic thin film. The thermal recording layer develops or changes a color or is melted and removed by heat of the infrared absorbing layer. Further, Pat. Document 1 discloses a recording method in which an infrared absorbing layer is caused to generate heat by irradiation of infrared laser light, and a thermal recording layer develops or changes a color or is melted and removed by this heat.
Japanese Patent No. 2561098 relates to a laser beam recording apparatus for performing image recording on a heat mode recording material, which comprises first and second semiconductor lasers for emitting laser beam spreading in a direction perpendicular to a pn junction plane and having an elliptic cross-sectional shape, a deflection beam splitter for combining the laser beams emitted from the semiconductor lasers, and a scanning optical system for scanning by using the laser beam combined by the deflection beam splitter. In the laser beam recording apparatus disclosed in Japanese Patent No. 2561098, the laser beam emitted from the first semiconductor laser and the laser beam emitted from the second semiconductor laser are combined with each other, and the semiconductor lasers are arranged in such a manner that a center of the combined laser beam is shifted to one end side in a major axis direction of a cross-sectional shape of one of the laser beams. Further, Pat. Document 2 discloses that main scanning is performed by the scanning optical system in a state where the center of the combined laser beam is positioned on the rear side in the direction of movement in the major axis direction of the cross-sectional shape of one of the laser beams.
However, in Japanese Patent No. 3266922, a laser having a high power output is required as a light source for outputting infrared laser light. For this reason, in Japanese Patent No. 3266922, even when a semiconductor laser small in size and relatively low in price is used, it is a fact that the output is limited to several watts with this semiconductor laser, and a recording speed of the line-type thermal head class cannot be realized. There is a method in which for example, a YAG laser or the like having an output equal to or larger than several tens of watts is used. However, when a YAG laser or the like is used, the price is higher than the semiconductor laser, and the apparatus becomes larger.
In Japanese Patent No. 2561098, the shapes of the laser beams emitted from the first and second semiconductor lasers are elliptic on the recording surface of the heat mode recording material, and are perpendicular to each other in the major axis directions. For this reason, the power of one semiconductor laser having the major axis in the main scanning direction of the laser beam is used for heat recording. However, the power of the other semiconductor laser having the major axis in the sub-scanning direction is not effectively used for heat recording in a part other than a part in which the other semiconductor laser overlaps with the one semiconductor laser. Further, in Japanese Patent No. 2561098, the laser beams are combined with each other by the deflection beam splitter, and hence the number of laser beams to be combined is limited to two.
An object of the present invention is to provide a contactless optical writing apparatus which can resolve the problem of deficient power at the time of thermal recording on a thermal recording medium by effectively utilizing power of a laser beam and can realize enhancement of the recording speed.